The growing trend towards the progressive shrinking in size of microelectronic devices is forcing the scientific community to find new approaches to the manufacturing of increasingly smaller miniaturized devices.
One such approach is given by molecular electronics, which exploits the characteristic spontaneous arrangement of molecules in space for the construction of electronic devices. In particular complex molecules may be used as active devices by exploiting the self-assembling properties of such molecules to produce electronic devices on a nanometer scale.
Such an approach heavily relies on the full understanding of the electronic properties and behavior of the single molecules one intends to use.
Much experimental and theoretical work has been carried out in this field but, to date, no device has been obtained which can successfully measure or predict the electric behavior of a single molecule and thus evaluate its conduction.
Existing devices include the one described in patent application FR 2847983 (Centre National de la Recherche Scientifique): a miniature device for electrical detection of ligand-receptor interactions which comprises an electrically insulating solid support having a flat surface and carrying a matrix of nano-electrodes arranged in pairs. The inter-electrode space is covered with an organic hydrophilic layer for the fixation of polypeptide probes. The device is connected to a system which measures the electrical conductivity between each pair of nano-electrodes.
U.S. Pat. No. 6,870,235 (Abstreiter et al.) describes a nanometric semiconductor sensor device for the detection of target molecules and molecular interactions based on silicon on insulator (SOI) technology. The semiconductor biosensor comprises a field effect transistor (FET) into which an SOI is converted by means of a silicon conversion technology based on a lithography method. Such device enables the selective recognition or detection of biomolecules on the basis of their electrical responses.
The most significant hurdle that is encountered in the measurement of the conductivity of single molecules is their isolation. Another obstacle is given by the natural tendency of free-standing molecules to polymerize and bond with each other. Multiple free-standing molecules also have a tendency to bend, with the consequent alteration of their conduction properties.
The need therefore arises for an alternative approach to the conventional methods for the isolation and measurement of the electrical conductivity of individually free-standing molecules.
To this purpose, an appealing alternative to conventional photolithographic methods is given by diblock copolymers, which provides a low-cost and reliable way of patterning at nanometer-scale. The formation of nano-masks through the use of diblock copolymers relies on the spontaneous arrangement of these molecules to form a dense lattice of hexagonally arranged cylindrical pores across the depth of a thin film and over wafer-scale areas.
Self-assembled diblock copolymer thin films may also be used as sacrificial layers for the transfer of such dense nanoscale patterns into robust and thermally stable materials such as SiO2 layers. Other applications include the formation of pillars rather than holes by reversing the tone of the polymer template, which is obtained by selecting the complementary block molecular weight ratio.
To this respect, application JP2003129288 (Canon KK) describes a process for manufacturing a porous structure of regularly arranged pores wherein: a block copolymer layer is formed on the material to be processed; the block copolymer layer is thermally annealed to form sea and island structures; the island structures are removed from the block copolymer layer after the sea and island structures have been subjected to either anodic oxidation or anodic formation to form a nanoporous structure reflecting the structure of the block copolymer layer.
Patent application US2004144726 (Chmelka et al.) also describes a mesoscopically ordered, hydrothermally stable metal oxide-block copolymer composite or mesoporous material formed using amphiphilic block copolymers acting as structure directing agents for the metal oxide in a self-assembling system.
Patent application US2003185985 (Bronikowski and Hunt) describes a method of forming a periodic array of nano-scale objects using a block copolymer. The method includes the steps of: depositing a block copolymer of at least two blocks on a substrate to form an ordered meso-scale structured array of the polymer materials; forming catalytic metal dots based on the meso-scale structure; and growing nano-scale objects on the catalytic dots to form an ordered array of nano-scale objects.
The doctoral thesis by Storm, A. J. “Single molecule experiments on DNA with novel silicon nanostructures” (ISBN 90-407-2485-7) describes the fabrication and use of nanostructures to study the physical properties of individual DNA molecules. In particular, there is a section of the thesis dealing with the electrical transport properties of individual DNA molecules in the dry state. Metal electrodes were fabricated spaced by 50 to 500 nm on silicon oxide. DNA molecules were deposited between these electrodes, and their electronic conductivity was measured using atomic force microscopy. In such thesis, the cooperative effect of at least 5 DNA strands connected in parallel was measured. It was concluded, under the chosen experimental conditions, that DNA was insulating at length scales larger than 40 nm.
As many experimental and theoretical studies, here reported, show, DNA conductance is an open debate. It is believed that DNA conductance can be strongly affected by both the spatial localization of the strand (whether it is straight or bended) and the cooperative effects exerted by the molecules.
In view of the above, the problem underlying the present invention is that of providing a device for the measurement of the electrical conductivity and quantum effects of individual molecules wherein the above cooperative effects can be isolated and wherein the difficulties and drawbacks of presently used such devices can be overcome.